Microwave Sintering Technology for The Production of Metal Oxide Varistors
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MICROWAVE SINTERING TECHNOLOGY FOR THE PRODUCTION OF METAL OXIDE VARISTORS G. McMAHON*, A. PANT*, R. SOOD*, A. AHM*AD AND R.T. HOLT*** *Ceramics Kingston Inc., P.O. Box 655, Kingston, Ontario, Canada K7L 4X1 **Ceramic Section, EM&R/CANMET, 405 Rochester St., Ottawa, Ontario, Canada KIA OGI ***National Aeronautical Establishment, NRC, Building M-13, Montreal Rd., Ottawa, Ontario, Canada KlA 0R6 ABSTRACT A microwave sintering technology has been developed for the production of metal oxide varistors. The electrical properties (leakage current and non-linearity coefficient) of the microwave sintered devices were found to be comparable to those obtained for conventionally sintered varistors of identical composition. Additionally, the reference voltages were greater (by a factor of two) and the biaxial moduli of rupture were higher for the microwave sintered specimens. These differences have been attributed to the smaller grain size associated with the microwave sintered specimens. INTRODUCTION Owing to their non-ohmic current-voltage characteristics, metal oxide varistors are being used to protect electronic and electric components against voltage transients or surges. These devices are normally connected in parallel with the component to be protected. Under normal operating conditions, the varistor exhibits a high resistance such that almost all the line current is drawn by the component. However, when a certain voltage is exceeded as a result of a surge, the varistor becomes highly conducting, and the surge current is thus diverted through the varistor, protecting the component. These devices are normally fabricated from suitably mixed powders of ZnO doped with several other metal oxides (Bi 2 0 3 , Sb 2 0 3 , Co 3 0 4 , MnO 2 , NiO and A1 2 0 3 ). The mixed powder is coldpressed into pellets of the required shape, sintered and annealed using high temperature kilns. The desired electrical properties are obtained by carefully controlling the powder composition and the sintering and annealing schedules. In order to avoid cracking in the pellet as a result of thermal stresses, slow heating rates up to the required final temperature are necessary. The technology currently being used to manufacture metal oxide varistors thus involves very lengthy sintering schedules and annealing steps, resulting in low equipment productivity. One possible method to alleviate these problems is through the use of microwave energy in the sintering and annealing steps. One of the many advantages [1] attributed to microwave energy in ceramic processing is that heat is generated within the material itself and heating is thus volumetric in nature. In conventional processes, heat is supplied from external sources, and heating is therefore dependent on thermal conduction from the ceramic surface. Hence, through use of microwave energy, samples may be brought to temperatures rapidly without the adverse thermal stresses associated with conventional heating, and they are additionally predicted to contain a more uniform microstructure [2]. Mat. Res. S
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